What Is Photosynthesis?
The process plants use to convert sunlight, water, and carbon dioxide into food (glucose) and oxygen.
What It Is
Photosynthesis is the process by which plants, algae, and certain bacteria convert sunlight, water, and carbon dioxide into glucose and oxygen. It happens inside chloroplasts, driven by the pigment chlorophyll, and can be summarized as: light energy plus water plus carbon dioxide produces sugar plus oxygen. You can spot references to it any time a text discusses plant energy, oxygen production, or the base of a food chain. It matters because it is the foundation of almost all life on Earth: every calorie you consume traces back to energy originally captured from sunlight through this process.
Need Help With Science Concepts?
Our subject-specialist science tutors cover SNC1D/SNC2D foundations through Grade 11-12 Biology, Chemistry, and Physics — aligned to the Ontario curriculum.
Examples
Why Leaves Are Green
Leaves are green because of chlorophyll, the pigment that absorbs the light energy needed for photosynthesis. Chlorophyll absorbs red and blue light very effectively but reflects green light — and reflected light is what we see. In autumn, when days shorten and temperatures drop, many trees stop producing chlorophyll. As the green fades, the yellow and orange pigments that were always present in the leaf become visible, which is why deciduous forests turn colour in the fall.
The Oxygen We Breathe
The oxygen in Earth's atmosphere is almost entirely a product of photosynthesis. For the first two billion years of Earth's history, the atmosphere had almost no free oxygen. Then photosynthetic bacteria called cyanobacteria began releasing oxygen as a waste product, gradually transforming the atmosphere into what it is today. Every breath you take is made possible by photosynthesis — past and present. Rainforests, ocean phytoplankton, and even the grass in your backyard are continuously replenishing the oxygen supply.
Photosynthesis and Climate Change
Because photosynthesis pulls carbon dioxide out of the atmosphere and locks it into plant matter, plants and forests act as carbon sinks — natural systems that absorb more carbon than they release. This is why deforestation is a significant driver of climate change: when forests are cleared and burned, the carbon stored in those trees is released back into the atmosphere as CO₂. It is also why reforestation and the protection of old-growth forests are among the most straightforward tools available for reducing atmospheric carbon.
Start Your Child's Learning Journey
Have questions or ready to get started? Reach out to our team and let's discuss how we can support your child's learning.
Tell us about yourself
We'll use this to get in touch with you.
Step 1 of 3